python3: enforce byte prefix for vfs.open()
Changeset ea8da5aa23c6e723d7f0006387a1a20eef58fc9a added raw prefix to open(),
but also to vfs.open(). The latter only handles bytestring arguments.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate -I 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null